Sleep-disordered breathing (SDB) encompasses a group of disorders where the breathing pattern or quality of ventilation is abnormal during sleep. Obstructive sleep apnea (OSA), the most common such disorder (affecting a possible 4-50 of the adult population), is characterized by repetitive closing or collapse of the upper airway and partial or complete diminution of breathing. The obstruction is normally ended by the patient arousing briefly when the muscles of the upper airway act to clear the obstruction. The treatment of choice for OSA is continuous positive airway pressure (CPAP) as first described by Sullivan [Sullivan C E, et al. Reversal of obstructive sleep apnea by continuous positive airway pressure applied through the nares. Lancet 1981 Apr. 18; 1(8225):862-5].
CPAP devices are particularized for the patient through a process known as titration in which the minimum pressure necessary to keep the airway open is determined. Excessive pressure leads to patient discomfort and interrupted sleep. The minimum required pressure may vary during the patient's sleep cycle, and accordingly auto-titrating devices have been developed that monitor the patient's respiration to determine and adjust the pressure as appropriate. In addition, some devices provide dual pressure levels in an attempt to make the patient more comfortable, providing greater positive pressure during inspiration than during expiration. Such devices monitor the patient's respiration cycle in order to switch between pressure levels at the appropriate phase in the respiration of each breath. Depending on their conditions, different patients may experience different levels of discomfort depending upon how quickly and accurately the device tracks the patients' efforts. Simple threshold tests may fail when breaths are irregular, for example, during the presence of coughs, sighs and snores.
OSA is often accompanied by snoring. Thus many CPAP devices seek to determine the occurrence of snoring and to quantify it. For example, U.S. Pat. No. 6,840,907, issued Jan. 11, 2005 provides a device having a sensor array and processor, which is capable of analyzing snore. In particular it measures the snore amplitude of the patient by passing digitized snoring signals through a high pass filter with a low frequency cut-off of approximately 10 Hz, calculating the modulus of each resulting signal, summing all the moduli and passing the sum through a low pass filter with a high frequency cut-off of between 0.5 and 2 Hz. The processor also measures the harmonic purity of the patient's snore, that is, its closeness in form to a simple sine wave—on the assumption that a non-obstructive snore has a different degree of harmonic purity than an obstructive one. Further the system produces a measure of the harmonic stability of the patient's snore, that is, the accuracy with which one cycle of the snore signal matches its predecessor—on the assumption that a non-obstructive snore has a different degree of harmonic stability than an obstructive one.
U.S. Pat. No. 6,705,315 describes a CPAP apparatus having a sound transducer and a system that responds to sound indicative of snoring.
For effective delivery of treatment pressure in an auto-titrating CPAP device, the treatment should be applied at the first sign of impending obstruction. In many patients, an apneic (obstructive) episode is often preceded by a snore. Therefore, detection of snore is crucial to pre-emptive delivery of therapy.
Current snore detection/estimation technique requires extensive modeling of various patient air paths and resulting complex calibration. According to the prior art, snore is the residual noise in region 20-300 hz after removing an estimation of motor and other extraneous noise from the total noise estimate. To estimate motor noise, a model of noise is obtained empirically for every possible combination of air path elements. This is then utilized to arrive at patient snore during delivery of pressure therapy. Essentially Snore=Total Noise-Intrinsic Device Noise. The complex methods for calculating intrinsic device noise adds considerable cost to the production of flow generator and also to each incremental design change in air path of the flow generator.
What is needed is an economical, simplified method for detecting snoring in a patient receiving pressurized air from a CPAP device.